Method for increasing the resolution of a color television camera with three mutually-shifted solid-state image sensors

ABSTRACT

A process for increasing the resolution of color television solid state cameras provided with three semiconductor image sensors, each assembled of an array of optoelectric elements, includes: geometrically offsetting the sensors in horizontal direction by one third of the distance between the optoelective elements; phase-shifting scanning signals applied to respective sensors by 120°; generating standard signals for luminance and chrominance and matrixing an additional luminance signal; low-pass filtering the standard signals and high-pass filtering the additional luminance signal; and adding the filtered standard and additional luminance signals to produce a resultant luminance signal of higher resolution. A control signal is derived from the high-pass filtered additional luminance signal and is used for controlling amplitudes of the color-difference signals.

STATE OF THE ART

The invention is based on a process for increasing the resolution ofcolor television cameras of the type which includes threesemiconductor-image sensors, each sensor including an array ofoptoelectric sensing elements spaced apart one from the other inhorizontal and vertical directions by an equal distance, means forscanning line-by-line said sensing elements in the respective sensors toproduce output signals corresponding to three different colors, theposition of a first sensor being offset in the horizontal direction byone third of the equal distance relative to a second sensor and by twothird of the equal distance relative to a third sensor, the scanningmeans including means for generating three scanning signals applied tothe respective sensors, a scanning signal for the second sensor beingphase shifted by 120° and a scanning signal for the third sensor by 240°relative to a scanning signal for the first sensor, means for matrixingthe output signals from the respective sensors to generate a standardluminance signal and standard color-difference signals, and means forlow-pass filtering the standard signals.

In contrast to video camera tubes, the semiconductor image sensorsrepresent a scanning system wherein the illuminated optical informationis scanned in two dimensions by discrete image spots. The resolution ofthe generated video signal is therefore calculated in accordance withthe scanning theorem, namely directly by the number of image spots to berealized on the image sensor.

Therefore, an increase in the resolution is only possible by atechnically very problematic increase of the number in image spots ofthe semiconductor-image sensor.

However, after an accurate testing of the signal frequency spectrum atthe output of a semiconductor-image sensor possibilities are availableto increase the resolution by a suitable signal processing withouttechnological measures. The frequency spectrum is the one of a pulseamplitude modulated signal which consists of a component in the baseband as well as components which are grouped by a multiple of a carrierfrequency. The actual limiting of the resolution in accordance with thescanning theorem results from the fact that the carrier frequency signalcomponents generate signal components in the base band which falsify thebase band signal. However, if it is possible to eliminate the carrierfrequency components a higher resolution is made possible, even with alimited amount of image spots.

A solid state television camera with a plurality of semiconductor-imagesensors is already known from West German Offenlegungsschrift 28 46 869,wherein the resolution of the television camera can be also improved bymeans of a suitable geometric disposition of the semiconductor-imagesensors and corresponding signal processing. However, the proposedsignal processing serves, primarily for shifting the phase of the outputsignals of the image sensors in accordance with their horizontaloffsetting with respect to each other and this circuit is veryexpensive.

It is therefore an object of the invention to provide a process forincreasing the resolution of television cameras with semiconductor-imagesensors, wherein the signal processing can be made simpler and performedmore effectively.

ADVANTAGES OF THE INVENTION

The process in accordance with the invention includes the steps of theuniformly weighting and adding said output signals from the respectivesensors to generate an additional luminance signal; high-pass filteringsaid additional luminance signal, adding the high-pass filteredadditional luminance signal to said standard luminance signal to producea resultant luminance signal; and deriving from said high-pass filteredluminance signal a control signal for said standard color-differencesignals. The process is advantageous in that it is not only lessexpensive but any interfering components which still may be present canbe practically completely eliminated.

Particularly advantageous is the use of a synchronous demodulator forshunting off the control signal from the high pass filtered additionalluminance signal, whereby it is assured that real color being present isnot suppressed at high luminance frequencies, while during a black/whiteoscillation a false chrominance from alias frequencies is prevented.

DRAWINGS

One exemplified embodiment for performing the process in accordance withthe invention is illustrated in the drawings and is explained in moredetail in the subsequent description, whereby only these parts areillustrated which are important for the invention. The drawings show:

FIG. 1 a block circuit diagram for performing the process in accordancewith the invention,

FIG. 1a a block circuit diagram of a part of the color matrixillustrated in FIG. 1,

FIG. 2 frequency spectra of the video signals shown in FIG. 1,

FIG. 3 spectra and time sequences of the signals in a color or ablack/white format.

FIG. 1 illustrates a part of a color television camera with threesemiconductor-image sensors 1,2,3 including corresponding color filters(not illustrated) for generating a green signal G, a red signal R and ablue signal B of a single image focused on the respective image sensors1,2,3. Each of the three image sensors 1,2,3 includes an array ofdiscrete image spots, whose geometric period or equal distance inhorizontal and vertical directions is τ_(H). Three output signals G,Rand B are phase-shifted due to a horizontal displacement of theindividual image sensors 1,2,3 against each other by a third of theimage spot period (τ_(H) /3), whereby base band components of the samephase and carrier frequency components of the first and second orderwhich are mutually phase shifted by 120°, are picked up on the terminals4,5,6. Thereby, the reading or scanning cycle control is performed bythree reading or scanning cycle frequencies which are phase shifted withrespect to each other by 120° and which are generated by a synchronizinggenerator 7, whereby these reading cycle frequencies are fed to theimage sensors 1,2,3 at control inputs 8,9,10.

The color signals G,R,B from the terminals 4,5,6 are fed to a colormatrix unit 11 in which, in a known manner, a standard luminance signalY=0.30R+0.59G+0.11B as well as standard color difference signals R-Y andB-Y are generated. These standard signals Y and R-Y or B-Y, whosescanning spectra are illustrated in FIGS. 2a and 2d, are fed throughcorresponding low pass filters 12,13,14.

The before described offset position of the three semiconductor-imagesensors 1,2,3 in the horizontal direction by which the obtainableresolution, as known, is triple of that of the individual image sensor,can be used to convert the luminance component of the color signal intoa modified luminance signal having a higher resolution. This presumesthat the output signals R,G,B of the three semiconductor-image sensorsmust be uniformly weighted and added in accordance withY'=0.33R+0.33G+0.33B, so as to generate an additional luminancecomponent. This addition is performed in the color matrix unit 11 bymeans of the circuit illustrated in FIG. 1a. The color signals G,R,Bwhich are present at the output terminals 4,5,6 are fed through assignedemitter follower stages 24,25,26 and through resistors 27,28,29 havingthe same value (for example 1 kΩ). The resistors 27,28,29 are connectedto one input of a difference amplifier 30, whose other input isconnected with ground by means of a resistor 31. The additionalluminance signal Y' is then picked up at the output 32. The branching ofrespective signals as indicated by arrows 34,35,36 and 37,38,39 in FIG.1a relates to the known further processing of these signals intostandard color difference signals R-Y,B-Y and into a standard luminancesignal Y. However, the additional luminance signal Y' generated in thecolor martrix unit 11 is incompatible with the standard colorproportioning which requires a luminance matrixing in accordance withY=0.30R+0.59G+0.11B. In order to satisfy the higher resolution, on theone hand, and also satisfy the color proportioning, on the other handthe additional luminance component Y' is fed through a high pass filter15 and is added to the standard laminance signal to produce a resultingluminance signal Ys in an adding stage 16. As seen from graphs (a) and(b) in FIG. 2, the 6-dB limit frequency fp1 of the low-pass filteredstandard luminance signal Y is equal to 6-dB limit frequency fp2 of thehigh-pass filtered additional luminance signal Y'.

The frequency spectra of the three signals Y,Y' and Ys are illustratedin FIGS. 2a, b, c. Due to the nonuniform weighting of R,G,B a so-calledsignal folding occurs in signal Y at the frequency range of allmultiples of the reading or scanning frequency f_(A) and the foldingsuperimposes the effective signal (0 to f_(A)). With a uniform weightingof R,G,B the additional luminance signal Y has the triple readingfrequency with a corresponding correction of distortion of the partialspectra. The combination of Y and Y' signals from low pass/high passfilters 12,15 produces in adder 16 only a residual folding error (crosshatched portion in FIG. 2c), which is superimposed on the Y_(s) signalup to the limit frequency f_(P1) of the low pass filter 12.

When recording pure black/white scene images an effect occurs wherebywith the increase of the geometric local frequency due to the offsetscanning (FIG. 2d), a color interference in the form of an aliasfrequency is folded to the given optical frequency around the reading orscanning frequency f_(A). Thereby, the luminance spectrum folds in thechrominance range around f_(A) and 2f_(A), while no alias components aregenerated around 0 and 3f_(A). In order to keep these chrominanceinterferences very small the low pass filters 13 and 14 are required forthe color difference signals R-Y and B-Y. Only one identical limitfrequency f_(P1) of the low pass filters 12,13 and 14 is meaningful forcolor proportioning purposes. In addition, this eliminates therequirement of the delay or transit time adjustment between theluminance and chrominance signals.

The limit frequency f_(P1) of the low pass filters 12,13,14 must besmaller than half of the reading frequency f_(N) (Nyquist frequency),since a luminance signal Y of this frequency results in a low frequencybeat of the alias components generated in the luminance channel as wellas in the chrominance channel around the reading frequency f_(A). Inparticular the chrominance folding lets a luminance component withNyquist frequency f_(N) (0.5f_(A)) appear to be very strongly disturbedon a false color background.

Nonetheless, lower chrominance frequencies which can pass through thedescribed low pass filters 13,14 of the color difference signalsR-Y,B-Y, arrive with a luminance signal Y for example in the proximityof the reading frequency f_(A). As long as the difference frequency ofthe luminance signal and the scanning rate is smaller than the low passlimit frequency f_(P1), strong color patterns appear in the image, whichcannot be differentiated in the chrominance channel from the genuinecolor information. Only information from the additional luminance signalY' behind the high pass filter 15 permits a possibility to distinguishbetween the appearance of genuine or folded chrominance. Therefore, acontrol voltage is derived from the signal amplitude generated behindthe high pass filter. The control voltage suppresses by means ofcontrolled amplifiers 17 and 18 arranged in the color differencechannels the possible alias frequencies in the chrominance signal duringthe appearances of luminance components above the low pass limitfrequency f_(P1).

By using a synchronous modulator 19 being operated at the frequencyf_(A) for deriving the control voltage from the additional luminancesignal Y' it is assured that with high luminance frequencies, which havean additional low frequency color background structure, the colorcontained in the real scene image is not suppressed, while in the caseof a black/white oscillation of a uniform frequency erroneouschrominance resulting from alias frequencies is prevented.

This will be explained with the assistance of FIG. 3 in an example of anoptical scene image which activates only the sensor for red with afrequency f₀ in the proximity of the reading frequency f_(A). Since onlyone image sensor 2 takes part in the signal scanning the offset isineffective, so that the additional luminance signal Y' as well as thestandard luminance signal Y have a spectrum which reproduces at thereading frequencies f_(A) (FIG. 3a). In this case, the additionalluminance signal Y' contains behind the high pass filter contains 15spectral components f₀ and 2f_(A) -f₀ which together represent a beatf_(S). After the synchronous demodulation a difference oscillationf=f_(A) -f₀ of the same frequency as the resulting low frequencyluminance component, is generated. An additional uniform chrominancevalue, which had been derived from the real scene, is fed to thecontrolled amplifiers 17,18 with a saturation modulation which issuperimposed by the present luminance interference. It is important thatthe interfering luminance structures do not become even more apparent bythe missing color. For example, in a red image all image details whichrepresent higher signal frequencies would be decolored and have a lightappearance.

However, if the same frequency f₀ appears as a pure black/white signal(FIG. 3b), all three semiconductor-image sensors 1,2,3 are taking partin the optoelectrical conversion and scanning and can contribute to analias improved Y'-signal. Instead of two frequencies f_(o) and 2f_(A)-f_(o) as described in the preceding example only one component iscontained at f_(o). This component also produces after the synchronousdemodulation an oscillation at the frequency f_(A) -f_(o), but only withhalf of an amplitude. This amplitude difference can be used as adecision making threshold for the controllable amplifiers 17,18, so asto let the chrominance pass in accordance with this example, while inanother case it is recognized as an alias structure and is suppressed.

The described process may also be operated in a modified form in thatonly two of the three available image sensors are brought into a 2-phaseoffset, preferably for red and green. In this case a resolution increaseby the factor of 2 is obtained for the additional luminance signal Y'.This solution is of interest because of the resulting interferencedistance, since the image sensor 3 for blue generally represents theleast sensitive image sensor which is not used for generating theadditional luminance signal Y'.

I claim:
 1. A method for increasing the resolution of a solid-statevideo camera wherein a single image is projected onto threesemiconductor image sensors, each sensor including an array ofoptoelectric sensing elements spaced apart one from the other in bothhorizontal and vertical directions by an equal distance, said cameraincluding means for scanning said sensing elements of said sensorline-by-line to produce respective output signals from said sensorscorresponding to three different colors, a first one of said sensorsbeing offset in the horizontal direction by one-third of said equaldistance relative to a second one of said sensors and by two-thirds ofsaid equal distance relative to a third one of said sensors, saidscanning means including means for generating one scanning signal foreach of said sensors, the scanning signal for said sensor beingphase-shifted by 120° relative to the scanning signal for said firstsensor and the scanning signal for said third sensor being phase-shiftedby 240° relative to the scanning signal for said first sensor, each ofsaid scanning signals having the same frequency, said camera furtherincluding means for matrixing said output signals from said sensors togenerate a standard luminance signal and standard color-differencesignals and means for low-pass filtering said standard luminance andcolor-difference signals, said method comprising the steps of:uniformlyweighting and adding together said output signals from said sensors togenerate an additional luminance signal; high-pass filtering saidadditional luninance signal; adding said high-pass filtered additionalluminance signal to said standard luminance signal to produce aresultant luminance signal; and deriving from said high-pass filteredadditional luminance signal a control signal for controlling theamplitudes of said standard color-difference signals.
 2. A method inaccordance with claim 1, wherein said control signal is derived fromsaid high-pass filtered additional luminance signal by means of asynchronous demodulator operating at said frequency of said scanningsignals.
 3. A method in accordance with claim 1, wherein the low-passfiltered standard luminance and color-difference signals have the sameupper limit frequency (f_(P1)).
 4. A method in accordance with claim 3,wherein said upper limit frequency (f_(P1)) of said low-pass filteredstandard luminance and color-difference signals is smaller than half ofsaid frequency of said scanning signals (f_(N) =0.5f_(A)).
 5. A methodin accordance with claim 4, wherein said upper limit frequency of saidlow-pass filtered standard luminance and color-difference signals is a6-dB limit frequency, and wherein said high-pass filtered additionalluminance signal has a 6-dB lower limit frequency (f_(P2)) which isequal to said 6-dB upper limit frequency (f_(P1)) of said low-passfiltered standard luminance and color-difference signals.